Portland, Ore. - Identifying toxins like the anthrax spores that were mailed to U.S. lawmakers in 2001 has been a process that can take hours to obtain preliminary results and days to get final confirmation. But investigators may soon be able to use a probe invented at Oak Ridge National Laboratory (ORNL) to make positive identifications instantly.
"Instead of extensively preparing samples before testing them, our nanoprobe can perform a direct analysis of samples. Also, because the nanoprobe is so small, we think it may eventually be used to detect individual molecules at the intracellular level," said Oak Ridge corporate fellow Tuan Vo-Dinh, who led the team that developed the nanoprobe at the Tennessee lab. Other team members included David Stokes and Zhenhuan Chi of the Life Sciences Division at Oak Ridge.
According to Vo-Dinh, the team is engineering the nanoprobe into a device the size of a shoebox and is adapting its technologies to such applications as homeland security, environmental monitoring, medical diagnostics and forensic analysis.
The nanoprobe leverages a common lab technique known as the surface-enhanced Raman scattering effect. Since the effect's discovery in 1928, the characteristic Raman scattering signatures of most known substances have been cataloged. The classic technique illuminates a sample with a sufficiently powerful monochromatic light and senses any lines that are added to the spectrum as a result of scattering in a transparent medium. But to make a medium like powered anthrax transparent to determine the scattering signature, the material must first be mixed with a special liquid.
"Our probe doesn't require that you prepare the samples,"Vo-Dinh said. "It will even work with dry samples, like powdered anthrax spores."
Today the preparation of samples often involves not just water, but a slurry that includes particles of silver mixed with the sample. The silver induces plasmons in the sample that amplify the Raman scattering effect.
Plasmons are collections of electrons that meld with photons to form a new order of object called a surface plasmon polariton. SPPs are collective oscillations of electrons at the boundary between a conductor and an insulator. They reverse the effect of a photonic crystal: Whereas crystals exclude light at special wavelengths (called the optical bandgap), SPPs enhance transmission in certain bands (the optical bandpass). Resonant SPPs in the sample accumulate electromagnetic energy and act as an antenna to amplify it.
Vo-Dinh and his colleagues reasoned that the plasmons could be produced by the probe itself. After testing several materials, the group settled on an optical fiber whose end was coated with a thin film of silver nanoparticles. In practice, Vo-Dinh said, a monochromatic laser beam is sent down a fiber-optic cable that has been sharpened to a 100-nanometer silver-coated tip. When the laser hits the silver nanoparticles, the silver electrons oscillate rapidly, yielding plasmons. The enormously enhanced electromagnetic field amplifies the Raman scattering effect so dramatically that even dry samples yield verifiable results in real-time.
Vo-Dinh's team has been using nontoxic samples for testing, but tests with toxins are in the offing. The team also plan to test the technique's suitability for disease identification in living cells, for other medical diagnostics and for forensic analysis.
Vo-Dinh's project at Oak Ridge has been funded by the Department of Energy's Office of Biological and Environmental Research and the Laboratory Directed Research and Development program.